Effect of surface roughness on femtosecond laser ablation of 4H-SiC substrates表面粗糙度对飞秒激光烧蚀4H-SiC衬底的影响

Ablation threshold is an important concept in the study of femtosecond laser micro- and nano-machining. In this paper, the ablation experiments of three kinds of surface roughness 4H-SiC substrates irradiated by femtosecond laser were carried out. The feature thresholds were systematically measured for three surface roughness SiC substrates and found in the modification and annealing regions ranging from coincidence (Ra=0.5 nm) to a clear demarcation (Ra=5.5 nm), eventually being difficult to identify the presence of the former (Ra=89 nm). Under multi-pulse laser irradiation, oriented ripple structures were generated in the annealing region, where deep subwavelength ripples (about 110 nm, Λ ≈ 0.2λ) can be generated above substrates with surface roughness higher than 5.5 nm. We investigated the effect of surface roughness on the ablation morphology, ablation threshold, and periodic structures of femtosecond laser ablation of 4H-SiC substrates, while the ablation threshold was tended to decrease and stabilize with the increase of pulse number N⩾500.